In recent years, the downscaling of optical systems has lead to a strong need for integration of a photoelectric device with an optical device, thus increasing the necessity and applicability of microlenses. A microlens may be integrated in a light source in order to efficiently combine optical fiber with the light source in an optical communication system, so that the entire packaging cost can be reduced. Also, since the microlens may be integrated in an optical detector to condense light into an active layer of the optical detector, the efficiency of the optical detector may be enhanced. Further, the microlens may be formed over a color filter of an image sensor to elevate the light sensitivity of the image sensor.
The necessity of the microlens in an optical interconnection system has been highlighted lately, and a vertical cavity surface emitting laser (VCSEL) is being watched with keen interest as an ideal light source for a parallel optical interconnection system because of its many structural advantages. Above all, a VCSEL with oxide current apertures has many strong points, for example, a low threshold current, high photoelectric conversion efficiency, and a single-mode operation, due to its small active region.
However, since laser beams irradiated from a surface are greatly diffused, when the VCSEL is applied to a free-space optical interconnection system, crosstalk may increase between channels, and an optical transmission distance and a tolerance in optical alignment are limited. Also, in a chip-to-chip optical interconnection system using an optical waveguide as a light transmission medium, combination efficiency between the light source and the optical waveguide is restricted by diffusion of laser beams. Therefore, by integrating the microlens into the photoelectric device, the packaging cost of systems can be reduced.
FIGS. 1 through 3 are cross-sectional views illustrating a conventional process of fabricating a microlens using the reflow of photoresist or polymer such as polyimide.
Referring to FIG. 1, a conventional method of fabricating a microlens includes depositing a polymer material 1 on a semiconductor substrate 2, patterning the polymer material 1, forming a cylindrical pattern of the polymer material 1 using a typical photolithography process as shown in FIG. 2, and reflowing the polymer material 1 by heating the resultant structure.
When the reflow process is finished, the microlens is fabricated as shown in FIG. 3 based on the property of the polymer material 1 that forms a curved surface due to surface tension. Alternatively, the polymer material 1 may be dry-etched as a lens type to form the microlens on the substrate 2.
Since a photoelectric device into which the above-described microlens is integrated can be applied to a system without optically aligning an external lens with the photoelectric device, the packaging cost of the system can be reduced, and the packaged system can be scaled down.
FIG. 4 is a cross-sectional view of a VCSEL into which a conventional microlens is integrated.
Referring to FIG. 4, the VCSEL includes a microlens 201 which is formed on a substrate 2 by reflowing or dry etching a polymer material. The use of the microlens 201 leads to a reduction in an emission angle of a laser beam 11. Also, the microlens 201 can be applied to an optical communication system or an optical interconnection system without an additional external lens.
The above-described VCSEL includes a p-type metal layer 3, an upper Bragg mirror 4, an aluminum oxide layer 5, an active layer 6, a lower Bragg mirror 7, and a current aperture 8.
However, the above-described method makes it difficult to fabricate a high-density microlens array. Specifically, when a distance between lenses is small, adjacent lenses are brought into contact with each other during a reflow process, so that a desired microlens array cannot be obtained. Also, since the microlens 201 can be fabricated only on the substrate 2, a bottom surface of the substrate 2 need to be polished to reduce scattering, and an anti-reflective coating (ARC) layer 10 should be coated on the substrate 2 and then the microlens 201 should be formed thereon in order to eliminate a Fabry-Perot resonator effect. Further, a photoelectric device 200 should be precisely aligned with the microlens 201 during a photolithography process, thus making an integration process complicated.